1. Field of the Invention
The present invention relates to the processing of printed documents and, particularly, to the addition of indicia comprising one or more colors to previously printed black on white documents. More specifically, this invention is directed to a document processing system having a single paper path and the capability of being interfaced with a high speed monocolor printer and, especially, to apparatus employing multiple print heads to add colored indicia to documents exiting an interfaced printer without reducing document throughput rate or requiring redirection of the documents exiting the interfaced printer into multiple processing paths. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention enables the addition of indicia, in selected colors, to printed documents exiting a high speed electrographic or xerographic printer, i.e., in a preferred embodiment the present invention is a high volume printer with accent color capability. There has been a long standing and unmet need in the art of the ability to provide color enhancement to conventional black on white printed documents. Previous attempts to satisfy this demand, as will be briefly described below, have been unsatisfactory in that there has been a failure to address the need to preserve the significant investment of potential users in their existing, installed black print data printers and, particularly, to recognize that this investment has been made with a primary objective of increasing printing speed.
As an example of the prior art attempts to achieve high volume printing with multiple color capability, two color printing capability has recently been added to conventional xerographic apparatus by using two developers, one for black and one for a single accent color, operating at different voltages. This approach, however, has the disadvantage that it cannot offer full spectrum color capability on a high speed data printer.
For users requiring or desiring more than a combination of black plus a single accent color, the only previous alternatives have been low speed systems characterized by high labor intensity and/or expensive investment in equipment. By way of example, a xerographic process employing multiple developers may be employed. Printers utilizing multiple developers are slow, typically five pages per minute maximum, and expensive. Ink jet printer technology also offers multiple color capability. However, the prior art ink jet technology employed water-based inks which imposed restrictions on the choice of paper being processed and, generally, presented problems with permanency as a result of moisture absorption. It is to be noted that ink jet technology is available which employs print media which is liquid in the jet and solidifies upon impact, such media typically being wax based. While the use of wax based inks provides excellent full color range, previously available printers employing this technology are notoriously slow.